In conventional Idle Air Control Valves (IACV) for vehicles, two lubricants are used in the core motor of the IACV, bearing oil and shaft grease. The conventional bearing oil is polyphenylmethyldimethylsiloxane fluid and is used in the open ball bearings of the IACV. The conventional shaft grease is a fluid grade silicone thickened with lithium soap that lubricates the shaft/rotor lead screw and the front journal bearing of the IACV. The IACVs manifold ambient environment specification is defined −40° C. to +125° C., typical heat for automotive under-hood parameters. The silicone based oil and grease are at times suitable when the IACV is mounted external to the intake manifold of a vehicle's engine.
A recent application of the IACV changed mounting from external to the intake manifold to insertion into the manifold. This application change led to a packaging change of the core motor. The IACV's zinc die cast housing was replaced with a plastic sleeve. The material change combined with the insertion into manifold reduced the heat transfer capabilities of the core motor. The core motor is an electrical device that generates internal heat when operational. CAE analysis calculated the IACV's ball bearing temperature reaches upwards of +200° C., due to internal heat build up and heat transfer losses due to mounting.
During testing at a dynamic thermal life cycle −40° C. to +125° C., several core motors lost functionality. Brown crystalline residue was found in the ball bearings. It was unknown if this residue was from material thermal degradation, contamination, or the phase change of the silicone fluid to silica solid. Discoloration of the shaft grease was also observed but it was not determined if the color change is degradation, contamination, evaporation, separation or absorption of by-products. Moisture has a major destructive influence on lithium soap grease. Root causes affecting bearing seizure are believed to be a combination of high temperature and moisture condensate in a dynamic (cycling) environment.
The conventional silicone base lubricants used in the IACV have a thermal functionality range of approximately −75° C. to +200° C. When the thermal capabilities of silicone fluids are exceeded, polymerization can occur such as physical change of the silicone fluid to silica solid. When polymerization occurs, the lubricant breaks down failing to provide the lubrication barrier between moving surfaces.
As is evident from the test results and the apparent thermal limitation of the production lubricants, there is a need for an alternate lubricant for an IACV to ensure the warranty capability of the IACV motor.